The present embodiments relate to a frequency modulator for provision of a frequency-modulated communication signal.
FIG. 1 shows a frequency modulator 1. The frequency modulator 1 has a modulator input 2 to which an unbalanced-to-earth or ground modulation source 3 for the provision of a modulation signal UM is connected. The modulation signal UM is an alternating signal (e.g., an alternating voltage). The modulation signal UM may also be a direct voltage in certain cases. The modulator input 2 is coupled via a resistor 4 to a node 5, to which two capacity elements 6, 7 of a capacitive element pair are connected. Both capacity elements 6, 7 are configured as capacity diodes in the example illustrated in FIG. 1 but may also, for example, be controllable capacitors. Alternatively, one of the capacity elements 6, 7 may be a capacity diode, while the other capacity element is a simple capacitor. The capacity diodes 6, 7 are connected to each other reverse-connected so that the respective cathodes are directly connected to the node 5. The anode of the capacity diode 7 is directly connected to a reference potential 8, while the anode of the capacity diode 6 is coupled to a resonator of an oscillator unit 10. The anode of the capacity diode 6 may be connected to the reference potential 8 via a coil of the resonator. Alternatively, if a quartz resonator is used instead of an LC resonator, a connection, in parallel to the quartz, of the anode of the capacity diode 6 to the reference potential 8 is sufficient (e.g., via a resistor connected in parallel to the quartz). The oscillator unit 10 together with the capacity diodes 6, 7, for example, form a voltage-controlled oscillator (VCO). The oscillator unit 10 is connected to the reference potential 8 and has an oscillator output 11 that is coupled to a modulator output 12. A frequency-modulated communication signal UK is provided at the oscillator output 11 and therefore at the modulator output 12.
The oscillator unit 10 may include an oscillator circuit that, for example, may have an oscillator and an oscillating circuit such as, for example, an LC-oscillating circuit, a quartz-oscillating circuit or the like.
In the absence of the modulation signal UM, a bias voltage is applied to the modulator input 2 of, for example, 2V (e.g., with regard to the reference potential 8). Thus, the two capacity diodes 6, 7 are operated in a reverse direction because a positive voltage drops between the respective cathode and the assigned anode. In the absence of a modulation signal UM, the frequency of the communication signal UK is therefore constant. If the bias voltage at the modulator input 2 overlaps the modulation signal UM, for example, such that at the modulator input 2, a sinusoidal signal is provided as a modulation signal with an amplitude of, for example, 1V, and consequently, a total alternating voltage of 1V to 3V is provided, the capacities of the capacity diodes 6, 7, and consequently, the oscillation frequency of the entire VCO alter. According to the variable frequency of an alternating voltage US at the oscillator unit 10, the same unit 10 then produces the communication signal UK. The frequency of the communication signal UK is varied analogously to the modulation signal UM. The frequency modulation of the communication signal UK is thus specified by the modulation signal UM that is produced with the aid of the modulation source 3.